nntp: avoid uninitialized vars from bogus LIST args

nntp: avoid uninitialized vars from bogus LIST args

We can only use the `list_' prefix in NNTP.pm for subcommands
which we intend to dispatch from client requests.  So prefix
the list_*_i subs with a `_' to prevent `args_ok' from firing
on a non-prototyped subroutine.

I noticed this problem in the previous change to reduce code
duplication between LIST subcommands.